1. Technical Field
The present invention relates to fiber optic communications and, in particular, to a multimode optical amplifier for use mode-division multiplexing systems.
2. Description of the Related Art
Advances in optical coherent detection and signal processing have led to tremendous growth in the spectral efficiency achieved in fiber. For example, 100 Tb/s transmissions at a spectral efficiency of 11 b/s/Hz are possible over a single-mode fiber. Owing to the nonlinear refractive index of silica—the primary material used in fiber optic cables—it is impossible to continue increasing spectral efficiency indefinitely by merely increasing the launched power. Even if the transmission medium were linear, Shannon's formula C=B log2(1+P/N), where C is the channel capacity, B is the bandwidth of the channel, P is the total received power, and N is the total noise, shows that capacity only scales as the logarithm of signal-to-noise ratio. As a result, high spectral efficiency is power inefficient.
Space-division multiplexing (SDM) may be employed, where data is carried over a plurality of parallel channels, which follows similar trends in other fields such as computing and wireless communications. For example, in response to saturating clock speeds achieved in integrated circuits, computer engineers have used multi-core processors. Similarly, wireless communications have used multiple-input multiple-output (MIMO) antennas, with the achievable capacity increasing as the number of independent “eigen-channels”, which is related to the maxima of the number of antennae employed at the transmitter and receiver.
In optical fiber transmission, two types of SDM schemes are available: (i) multicore fibers (MCF), where a single strand of glass fiber has a plurality of single- (or multi-) mode cores with low coupling, each capable of guiding optical signals; and (ii) multimode fibers (MMF), where a single strand of fiber has one core with sufficiently large cross-section area to support a number of orthogonal guided modes. Owing to the lack of available inline amplifiers, however, all MCF and MMF experiments to date have been single-span. To date, no attempt has been made to precisely control the gains seen by different propagating modes of an MMF at a given signal wavelength. On the contrary, MMFs have traditionally been used in a single-mode manner, with efforts being made to provide amplification for only one mode.